easy way: use 2 colors and mix them together, for example blue and orange, don't use black, dark blue is your darkest colour;
more hard way: take more wide palette, for example to our orange we can add more yellow in lights and more reds in dark points.
hardest way: use many colors and mix them in the one picture - usually it's a hard and long time process of many experiments at start stage of picture. Try make interesting colors in thumbnail and after that give details. I say again - do not use black in the sky (except star-sky), lights going to yellow and after white, darks going to red and after darkblue and violet. Hardest thing here is to find ballance
it's about of wizard kids, who say always young and if they diyng(was killed for example) they born again in the random place of the world, and the friends must find killed wizards and cast on the recalling spell, or they die again.... it's hard to describe with little words, I want say about name of the picture - they are try revenge with their killers and killers of their friends(but enemys will reborn again - it's endless war) ) The most powerfull mage of them is on the right(on that picture)